The present invention relates to the art of transmission of electrical signals, particularly telephone signals, through lines having internal resistance, for the purpose of powering relays, switching equipment or the like.
Telephone subscriber stations are electrically connected to centrally located switching equipment which, in response to electrical signals caused by the operation of equipment associated with the subscriber station, perform the function of placing the subscriber station into electrical communication with either another desired subscriber station or with further switching equipment which places the subscriber station into electrical communication with another desired subscriber station. The voltage source for the electrical signals is located at the central office, and electrical current flows between the subscriber stations and central office along loops having an internal resistance. At the telephone subscriber station, the loop is interrupted by an open circuit which is only closed in response to the operation of the control and selection functions of the subscriber station. By way of example, placing a telephone off-hook causes current to flow in the subscriber loop for the purpose of signaling the central office that a line is desired. When a rotary-dial telephone is dialled, the make-break dial action causes "dial pulses" of current to flow through the subscriber loop to actuate switching equipment at the central office. In either case, the flow of current in the loop provides electrical signals which operate equipment at the central office.
The electrical signals in the subscriber loop must have sufficient power to be capable of powering the necessary relays or the like at the central office. However, the transmission of electrical current from the central office switching equipment through the subscriber loop to the subscriber station and back to the central office switching equipment results in a power loss due to the distributed internal resistance of the loop itself. Thus, the length of the loop and the gauge and material of the line combine to create a distributed internal resistance which imposes practical limitations on the length of the subscriber loops that can be used to connect subscriber stations to the central office, on the types of materials that can be used for subscriber loops, and also the cross-sectional diameter, i.e., gauge, of the subscriber loops that can be used between the central office switching equipment and a subscriber station located a given distance from the central office. The effects of this practical limitation may be to preclude extending telephone service to a prospective subscriber, or to require subscriber loops which are either of a larger and most costly gauge or are made from a more costly material.
It has been known in the art to install a device known as a "loop extender" at some location in a subscriber loop to permit connection of a subscriber station which would ordinarily be located at too great a distance to provide reliable signals to the central office switching equipment. A common type of prior art loop extender is illustrated by the voltage booster devices described in U.S. Pat. Nos. 3,872,258 and 3,757,052. These devices increase the voltage on the loop, thereby providing the potential for an increased current level in the "extended" loop.
The voltage booster loop extenders described above presented a number of problems in their ordinary commercial use. Initially, the voltage booster loop extenders of the prior art tend to be expensive and add considerably to the cost of installation of a subscriber station at a long distance from the central office. Further, the voltage booster loop extenders required elaborate circuits to permit reverse-polarity or "reverse battery" supervision at the central office to cause a reverse current to flow in the loop.
Additionally, more sensitive relays have been used in the past in the switches of the central office equipment. However, the substitution of more sensitive relays may also be costly, and affects the uniformity and interchangeability of the central office switching equipment.